Siemens SIMATIC S7-400 Manual page 135

Example: 20% communication load
You have configured a communication load of 20% in the hardware configuration.
The calculated cycle time is 10 ms.
A 20% communication load means that, on average, 200 μs and 800 μs of the time slice
remain for communications and the user program, respectively. The CPU therefore requires
10 ms / 800 μs = 13 time slices to process one cycle. This means that the actual cycle time
is 13 times a 1 ms time slice = 13 ms, if the CPU fully utilizes the configured communication
load.
This means that 20% communications do not increase the cycle linearly by 2 ms but by
3 ms.
Example: 50 % communication load
You have configured a communication load of 50% in the hardware configuration.
The calculated cycle time is 10 ms.
This means that 500 μs of each time slice remain for the cycle. The CPU therefore requires
10 ms / 500 μs = 20 time slices to process one cycle. This means that the actual cycle time
is 20 ms if the CPU fully utilizes the configured communication load.
A 50% communication load means that 500 μs of the time slice remain for communication
and 500 μs for the user program. The CPU therefore requires 10 ms / 500 μs = 20 time
slices to process one cycle. This means that the actual cycle time is 20 times a 1 ms time
slice = 20 ms, if the CPU fully utilizes the configured communication load.
This means that 50% communications do not increase the cycle linearly by 5 ms but by
10 ms.
S7-400 Automation System, CPU Specifications
Manual, 10/2006, 6ES7498-8AA04-8BA0
Cycle and Response Times of the S7-400
9.4 Communication Load
9-9